A detailed account of SN Ia models is beyond the scope of our general review of SN Ia spectra (for the latest discussions, see [1]}, [2]}, [3]}, [4]}, [5]}). However, in order to understand the context by which observations are taken and synthetic comparisons made, here we only mention the surface layer of matters relating to observed spectra. For some additional recent modeling work, see [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]}, [17]}, and [18]}.
